The present invention generally relates to perpendicular magnetic recording media, and, more particularly, to a perpendicular magnetic recording medium that can achieve a high recording density.
Conventionally, in-plane magnetic recording media have reduced media noise and secured the S/N in reproduction signals, so as to achieve higher recording densities to compensate decreases in output voltage due to high-density recording. In recent years, perpendicular magnetic recording media have become popular in pursuit of even higher recording densities.
FIG. 1 illustrates a conventional perpendicular magnetic recording medium 10. As shown in FIG. 1, the perpendicular magnetic recording medium 10 has a substrate 11. On the substrate 11, the perpendicular magnetic recording medium 10 has a soft-magnetic backing layer 12, a non-magnetic intermediate layer 13, a recording layer 15, and a protection layer 16 stacked in this order. The recording layer 15 is a magnetic film of a CoCr-based alloy formed by a sputtering method. Such a magnetic film is made up of the boundary of crystal grains of a high Cr concentration and crystal grains that are the cores of the crystal grains of a high Cr concentration, with Cr atoms segregating on the boundary surfaces of the crystal grains. Among these crystal grains, there are magnetostatic interactive effects and exchange interaction effects. The grain size of each of the crystal grains is large. If the distance between each two crystal grains is long, those interaction effects become greater, resulting in an increase of medium noise. To solve this problem, the materials and film-forming conditions for recording layers and underlayers have been optimized, so that the grain sizes can be reduced and made uniform.
However, recording layers of CoCr-based alloys formed by a sputtering method cannot satisfy today's demand for recording densities higher than 775 Mbits/cm2 (500 Gbits/inch2), because the grain sizes cannot be reduced and made uniform sufficiently. As a result, a sufficient reduction of medium noise cannot be achieved.
As a solution for achieving minute and uniform ferromagnetic crystal grains, a variety of chemical techniques have been suggested. These techniques are disclosed in publications such as Science (Vol. 287, No. 17 (2000), pages 1989–1992, Sun, et. al) and J. Mag. Soc. Japan (Vol. 25, No. 8 (2001), pages 1434–1440).
In accordance with the inventions disclosed in those publications, the spherical magnetic nanoparticles have grain sizes of nanometers. FIG. 2 is a sectional view of a magnetic recording medium 20 having stacked spherical magnetic nanoparticles. As shown in FIG. 2, a recording layer 25 and a protection layer 26 are stacked on a substrate in this order. The recording layer 25 has a thickness of 20 nm to 100 nm, and is formed by stacking spherical magnetic nanoparticles.
Although the recording layer 25 shown in FIG. 2 is formed by uniform magnetic nanoparticles 27, the positions of the magnetic nanoparticles 27 are shifted on each layer in the film thickness direction if the magnetic nanoparticles 27 have a meticulous filling structure. As a result, the magnetic transition regions are disturbed at the time of recording. Because of this, the recording layer 25 cannot achieve a sufficient reduction of medium noise.
To reduce medium noise, a perpendicular magnetic recording medium 30 having conventional spherical magnetic nanoparticles in the form of a single layer has been suggested. As shown in FIG. 3, the perpendicular magnetic recording medium 30 has a substrate 31. On the substrate 31, the perpendicular magnetic recording medium 30 has a soft-magnetic backing layer 32, a non-magnetic intermediate layer 33, a recording layer 35, and a protection layer 36 stacked in this order. The recording layer 35 is formed by spherical magnetic nanoparticles 37 that are aligned at uniform intervals and formed into a single layer. In this structure, the unevenness of the magnetic nanoparticles in the film thickness direction is eliminated, and the exchange interaction effect can be reduced. Accordingly, this perpendicular magnetic recording medium 30 can reduce the medium noise and achieves a higher recording density.
With the recoding layer 35 of the perpendicular magnetic recording medium 30, however, there is a problem of thermal instability. More specifically, since the exchange interaction effect is restrained, the residual magnetization rapidly decreases after a recording operation. It is a known fact that, to achieve thermal stability of residual magnetization, the index expressed as KuV/kT should be great. Here, Ku represents the anisotropic energy, V represents the effective grain volume (equivalent to the total volume of the magnetic nanoparticles coupled by the exchange interaction effect), k represents the Boltzmann constant, and T represents the absolute temperature. Since the recording layer 35 of the perpendicular magnetic recording medium 30 has a small exchange interaction effect, the effective grain volume V becomes equal to the volume of each one of the magnetic nanoparticles 37. As the volume V becomes smaller, the index KuV/kT also becomes smaller, resulting in thermal instability. Judging from these facts, the perpendicular magnetic recording medium 30 cannot achieve a sufficient reduction of medium noise and greater thermal stability at the same time.